1. Field of the Invention
The present invention relates to a medical information processor, an image photographing system, and an absorption coefficient calibration method, and more particularly to an information processor, an image photographing system and an absorption coefficient calibration system which are preferably used in order to carry out a beam hardening calibration.
2. Related Background Art
It is necessary to consider that X-rays emitted from a radiation source of an X-ray CT (Computed Tomography) apparatus are not X-rays of monochromatic energy but X-rays of multi-color energy. It is because when a sensor output value with respect to the X-rays after the penetration of a subject is detected and then a radiation absorption coefficient u of a X-ray penetration section is determined, a change in X-ray spectrum-caused by a penetration thickness of the subject has bad influence on a reconstruction image (tomographic image).
That is, as a photon becomes lower in energy in the X-rays, its attenuation becomes greater, and as the penetration thickness of the subject becomes larger, the X-ray material becomes further stiffened (a high energy component of a spectrum becomes relatively large). Thus, as shown in FIG. 22, a property 102 of the sensor output value that is ideally linearly changed with respect to the penetration thickness indicates a non-linear property 101.
Thus, even if a circular cylinder member 103 made of a uniform material shown in FIG. 23A is used as the subject and then data of a section 104 thereof is gathered to obtain the reconstruction image, the radiation absorption coefficient p becomes low in the center of the section 104 having a large penetration thickness. For this reason, as shown in FIG. 23B, the radiation absorption coefficient p ought to be originally distributed as indicated by a property 105. However, it is distributed as indicated by a property 106, and a tomographic image is consequently configured (such phenomenon is referred to as a beam hardening phenomenon).
Thus, unless the variation in the sensor output with respect to the X-rays after the penetration through the subject is suppressed, or unless the variation is monitored and the variation component and the attenuation property received at the time of the actual subject penetration are taken into consideration to carry out a proper data calibration, an accurate inspection cannot be executed. In this case, a method is usually employed in which the variation in the sensor output is monitored and a predetermined calibration corresponding to the variation in the sensor output with respect to the X-rays after the penetration through the subject and corresponding to the attenuation property received when the X-rays are penetrated through the subject is carried out.
Such method is required to obtain the attenuation property when the X-rays are penetrated through the subject, and the output values for various penetration thicknesses are gathered. Then, when the output values for the various penetration thicknesses are gathered, conventionally, phantoms having various penetration thicknesses are imaged, and output values are determined, as described in (Japanese Patent Application Laid-open No. 2003-000580) and (Japanese Patent Application Laid-open No. H03-026241)
However, in order to determine the attenuation property when the X-rays are penetrated through the subject, many data are required to be gathered. Also, if the phantoms whose penetration thicknesses are different, there is a limit to a data amount to be gathered. Thus, any penetration thickness cannot be selected, and an approximation to the penetration thickness different from the penetration thickness of the imaged phantom becomes very rough. Moreover, measuring the many data takes a longer measurement time corresponding thereto.
In this way, the conventional art has a problem in that at the time of the beam hardening calibration it is very difficult to obtain the data of any penetration thickness.